1. Field of the Invention
The invention relates to fiber-optical communication systems and processes for operating such systems.
2. Discussion of the Related Art
In non-solitonic optical communication systems, a variety of techniques have been used to reduce the distortions of optical pulses that are caused by dispersion and nonlinear optical effects. The list of techniques includes pseudo-linear transmission, dispersion mapping, and optical phase conjugation. These techniques are able to significantly improve the transmission capabilities of long-haul optical communication systems.
Pseudo-linear transmission (PLT) involves transmitting optical pulses under operating conditions that include: a bit rate of 10 Giga bits per second (Gb/s) or more, a wavelength of 1.25 micrometers (μm) to 1.7 μm, a pulse full width at half maximum power of 60 pico seconds (ps) or less, and a pulse duty cycle of between 10% and 70%. PLT operating conditions are, e.g., described in U.S. Pat. No. 6,542,678. PLT operating conditions are advantageous when transmission single-mode optical fibers (SMFs) have high dispersions, e.g., about +2 ps or more per nanometer (nm) per kilometer (km) at 1550 nm. Due to their high dispersion, such transmission SMFs produce substantial broadening of optical pulses. The broadening causes inter-pulse interactions to be averaged thereby reducing the total pulse distortion from inter-channel interactions.
Dispersion mapping involves controlling the cumulative chromatic dispersion along an optical communication path via lumped in-line dispersion compensating devices. The in-line, dispersion-compensation devices cause the cumulative dispersion to make abrupt jumps at the ends of transmission SMFs. The resulting map of the cumulative dispersion may be singly periodic, multiply periodic, or aperiodic over the optical communication path. These kinds of nontrivial dispersion maps can reduce distortion of optical pulses due to both chromatic dispersion and intra-channel nonlinear optical effects.
Useful nontrivial dispersion maps have, e.g., been described in U.S. Pat. No. 6,583,907 and U.S. Pat. No. 6,606,176. In addition, advantageous pre-compensations for some dispersion maps are described in U.S. patent application Ser. No. 10/152,645, filed May 21, 2002 by R.-J. Essiambre et al.
An optical phase conjugator (OPC) reverses the phase of propagating optical signals. Optical communications systems have used OPCs to compensate for the pulse distortions caused by nonlinear optical effects. In particular, such systems have positioned a single OPC at the midpoint of symmetric optical communication paths. For such an arrangement, the second half of the optical communication path removes the distortions of an optical pulse that were produced in the first half of the optical communication path.
While the above-described techniques have helped to reduce pulse distortions in non-solitonic optical communication paths, further improvements are desirable.